Clamping circuit



United States Patent 3,164,775 CLAMPING CIRQUIT Harry C. Morgan,Woodland Hills, Califi, assignor to North American Aviation, inc.

Original application Nov. 9, 1956, er. No. 621,397, new Patent No.3,071,762, dated Jan. 1, 1963. Divided and this application Jan. 26,1961, Ser. No. 89,601

7 Claims. ((31. 328-98) This invention relates to a circuit formaintaining the value of an analog signal for a desired time.

This application is a division of application Serial No. 621,397, filedNovember 9, 1956, now Patent No. 3,071,762. r i

An object of the invention is to facilitate conversion of one or morevariableitanalog-signal quantities into digital code at very high speed.

Another object is to momentarilyretain instantaneous magnitudes of avariable signal under cyclic control.

Other and further objects,-features and advantages of the invention willbecome apparent as the description proceeds,

In carrying out the invention in accordance with a preferred formthereof a clamping circuit is arranged to receive a continually varyinganalog input signal and momentarily, upon occurrence of a trigger orcommand,

- provide an output indicative of the value of such analog input signalat the instant of such trigger or command. This output is maintained atsuch value for a relatively short period of time which may be necessary,for example, to perform conversion of the analog input signal to digitalform. For this purpose there is provided a diode array with first andsecond resistors connecting ends of the array to points of fixedpotential comprising opposite sides of a direct current source. Switchesare provided for selectively transferring the input signal to the outputby momentarily shunting the diode array and respective ones of theresistors upon receipt of the trigger or command pulse by thefswitches.An input terminal for receiving the input analog signal is connected toone terminal of the diode array. A storage condenser and other inputcircuitry is connected to another terminal of the diode array. Duringthe momentary non-conduction of the switches. in response to the shortduration trigger pulse, the charge on the output condenser is causedtofollow the value of the input signal by virtue. of the connectionth'erebetween aiiorded by the diode array which is conducting. Then uponremoval of this momentary trigger the normally conducting switchesoperate to shunt the diodes and respectively shunt diiierent ones of theresistors whereby the potential on the condenser will remain fixeduntilthe next trigger pulse allows a charge of condenser potential tooccur again.

A better understanding of the invention will be afforded by thefollowing detailed description considered in conjunction wi-th theaccompanying drawings in which FIGS. 1 and 2 are graphs illustratingoperation of the recycling clamp; and

FIG. 3 is a circuit diagram of the clamp. 7

Like reference characters are utilized throughout the drawing todesignate like parts.

The function of the recycling clamp is illustrated in FIG. 1 of thedrawing in which the instantaneous magnitude of another variablequantity are represented by a curve 71 and the magnitudes of anothervariable quantity are represented by curve 72. Sampling portions of thecurves 71 and 72 are in successive time intervals, for example 100microsecond increments of time, would result in the composite curvehaving segments 73 and 74.

In order that the continuously varying analog quantities represented bythe curve segments 73 and 74 may be converted to digital code, thevalues of the curve'segments 73 and 74 at some predetermined point inthe curve or the average value thereof are detected and held during theperiod that the digital code is being produced. With the use of arecycling detector circuit of the type illustrated in FIG. 3, theinitial value of the curve segment 73 is held for an increment of time,for example 100 microseconds as represented by the horizontal line 75 inFIG. 1, :and the initial value of the curve 74 is held during the nextmicroseconds as represented by the hori-. zontal line segment 76. It isto be understood that other analog quantities are detected in successive100 microseconds time intervals.

Considering, for example, the variable quantity represented by the curve71, a voltage corresponding thereto produced, for example, by ananalog'transducer (not shown) is applied to the input terminal oftherecycling clamp in response to the action of a commutator under thecontrolof a triggered sweep generator as more particularly described inthe aforesaid Patent 3,071,762.

The circuit of FIG. 3 comprises a pair of'switching tubes 78 and 79shown as triodes which may constitute two halves of a twin triode tube,a cascode cathode follower comprising triode elements 81and 82 which maybe parts of a twin triode, and a diode array 83 which may consist eitherof thermionic discharge elements or semiconductors, for example,silicone diodes.

diode array 83 comprises four diodes 84, 85, S6 and 87 As shown, the

current flows from a terminal 88 to a terminal 89. The

terminals 88 and 89 are connected to a positive power supply terminal 91and a negative power supply terminal 92 through resistors 93 and 94,respectively.

The switching tube 78 is connected between the terminals 88 and 92 so asto bypass the diode array 83 and the diode resistor 94; whereas theswitching tube 79 is connected between the terminals 91 and 8 so as tobypass the diode resistor 93 and the diode array 83. The switching tubes78 and 79 have control electrodes or grids 5 and 96, respectively. Thecontrol elements 95 and 96 are coupled to a gate terminal 97 throughcoupling condensers $8 and 99. A triggered sweep generator (not shown)is arranged to supply a negative trigger pulse 1131 to the gate terminal97.

Input terminals 102 and 103 are provided at which the analog inputsignal from the commutator (not shown) or an analog transducer (notshown) is supplied. As shown, the analog input terminal 1% is groundedand the analog input terminal 1&2 is connected to the diode array 83.For example, where a four-diode bridge arrayis employed, the terminal162 may be connected to a junction terminal 1% of the diode array 83. Inthis arrangement, a second diode array junction terminal 105 isconnected to a control electrode 1% of the cascode cathode follower tube82. In the cascode cathode follower, the tubes 81 and 82 are connectedin series between the positive power supply terminal 91 and the negativeterminal 92. There is a cathode resistor 167 connected in the cathodeline of the tube 81 to the anode line of the tube 32 and the tube 82 isprovided with a cathode resistor 198. The tube 81 has a controlelectrode It)? connected to the anode 111 of the tube 82 for stabilizingthe tube 81. 7 An output terminal 112 is connected to the cathode 114 ofthe tube 82, and the ground terminal 103 serves as a second outputterminal. The cascode. follower tubes 81 and 82 provide a high degree ofstability by reason of the feedback employed.

The clamping circuit of FIG. 3 operates in the following manner: whenthere is no signal applied to the input terminals 102 and N3 theswitching tubes 78 and 79 are heavily conducting, pulling the potentialof the diode array terminal 88 well below that of the diode array s,1ea,

terminal 89. Consequently, the diodes S to 37 are cut off and a chargeupon a storage condenser 113 is isolated. Since the charge upon thecondenser 13) controls the potential of the cascode cathode followercontrol electrode 106, it determines the voltage output at the terminals112 and 103.

However, when the brief strong negative pulse ltll is applied to thegrids 55 and 96 of the switching tubes '78 and 79, they are cut off orrendered nonconducting. The voltage at this time is determined by thevoltage drops across the resistors 93 and 94, and the diodes 84 to 8?,which have become conducting, and the voltage is also modified by theanalog signal at the input terminm 102 which is connected to thejunction 16 iofv the diode array 83. Consequently, the voltage at thejunction terminal 105 and therefore at the output terminal 112 will riseand fall with the input signal.

The function of the recycling clamp of FIG. 3 is illustrated in FIG. 2in which the vertical lines 115 represent the times at which theswitching tubes 78 and "79 are rendered nonconducting. The dotted curve116 represents the analog signal curve. The analog value is read intothe clamp of FIG. 3'at each of the instants represented by verticallines 115 as shown by horizontal lines and line segments H7. The valueof the output voltage is preserved at points between read-out by thestorage condenser 113 of FIG. 3.

The full four-diode bridge array 33 of FIG. 3 provides cathode humbalance as well as cancelling out the effect of diode contact potentialsand their variations with changing heater voltage if thermionic valvesare employed. However, it is unnecessary to employ a four-diode arraywhen silicone diodes are employed. For example, the diodes 84 and 36 maybe omitted, in which case the input terminal 1912 may be connectedeither to the terminal 88 or to the terminal 89.

. For simplicity, if desired, the cathode follower 31 may also beomitted with the anode 111 of the tube 32 connected directly to thepower supply terminal 91 so as to employ a simple cathode followeroutput stage.

An important factor to be taken into account is the selection of thediodes and the condenser 1E3. After diode conduction, current carriersare left over which briefly (order of 1 microsecond or less) permitnegative current conduction. In order to avoid any possibility of thiselfect causing an error in the charge left on the condensers 113, thediodes are carefully matched for the negative conduction elfect so as tominimize the error. The accuracy of the operation is least affected byinput impedance when the terminal 38 (assuming connection there of theanalog input terminal 102), is balanced to ground potential.

The invention is not limited to the use of any particular constants ortubes. However, satisfactory operation is obtained where the switchingtubes 78 and '79 are elements of a type 5687 twin triode and the diodes84-87 are of the 1N2l4 type with 200 volt positive power supply for theterminal 91, and 200 volt negative power supply at the terminal 92, theresistors 93 and 5 4 constituting 66,000 ohm resistors and the resistor118 constituting a 1,000 ohm resistor. In the simplified circuit, withonly two triodes the resistor 93 may be 133,000 ohm, resistor 94 may be30,000 ohm, and the negative power supply voltage of the terminal 2 maybe minus 30 volts.

Although the invention has been described and illustrated in detail, itis to be clearly understood that the same is by way of illustration andexample only and is not to be taken by way of limitation, the spirit andscope of this invention being limited only by the terms of the appendedclaims.

I claim:

1.ln combination, a diode array, resistors in serieswith the array ateach end, for connection to a current source, an electronic switchacross the array and one rcsister, a second electronic switch across thearray and the other resistor, means for simultaneously operating bothswitches to non-conduction, an input signal terminal connected to thearray, and a condenser connected to the array and an end of one of theresistors, for storing the input signal magnitude between switchoperations.

2. A recycling clamp comprising in combination, a pair of switchingtubes, 21 cascode cathode follower, a diode array comprising four diodesconnected as a bridge, direct current power supply terminals withresistors interposed between the terminals and the diode array, thediode array having an intermediate terminal for receiving analog inputsignals, the cascode cathode follower having a control electrode, thediode array also having an intermediate terminal connected to saidcontrol electrode, the first switching tube having a control electrodefor response to trigger pulses and being connected across one of saidresistors and the diode array, the second switching tube having acontrol electrode connected to the control electrode of the firstswitching tube and being connected across the other of said resistorsand the diode array whereby a clamped voltage appears as an output fromthe cathode follower.

3. A recycling clamp comprising in combination input terminals forreceiving an analog signal, output terminals for supplying a clampedsignal value corresponding to an instantaneous value of the analogsignal, first and second direct currentsupply terminals, seriallyconnected diode team having first and second terminals and intermediateterminal means, a first resistor connected between the first directcurrent supply terminal and. the first diode means terminal, a secondresistor connected between the second diode means terminal and thesecond direct current supply terminal, a switching tube having a currentconductingelectrode connected to the first diode means terminal, acurrent conducting electrode connected to the second resistor, and acontrol electrode, a second switching tube having a current conductingelectrode connected to the first resistor, a current conductingelectrode connected to the second diode means terminal and a controlelectrode, a terminal for receiving trigger impulses coupled to saidcontrol electrodes, cathode follower means connected to the first andsecond resistors having a control electrode and having a cathoderesistor connected to the said output terminals, the control electrodeof said cathode follower means being connectedto intermediate terminalmeans of the diode means and the analog input terminal being connectedto intermediate terminal means of the diode means.

' 4. A recycling clamp comprising in combination input terminals forreceiving an analog signal, output terminals for supplying a clampedsignal value corresponding to an instantaneous value of the analogsignal, first and second direct current supply terminals, seriallyconnected diode means having first and second terminals and intermediateterminal means, a first resistor connected between the first directcurrent supply terminal and the first diode means terminal, a secondresistor connected between the second diode means terminal and thesecond direct current supply terminal, a switching tube having a currentconducting electrode connected to the first diode means terminal, acurrent conducting electrode connected to the second resistor and havinga control electrode, a second switching tube having a current conductingelectrode connected to the first resistor, a current conductingelectrode connected to the second diode means terminal and a controlelectrode, a terminal for receiving trigger impulses coupled to saidcontrol electrodes, cathode follower means connected to the first andsecond resistors having a control electrode and having a cathoderesistor connected to the said output terminals, the control electrodeof said cathode follower means being connected to intermediate terminalmeans of the diode means and the analog input terminals being coupled toone of the terminals of the diode means.

5. A recycling clamp comprising in combination input terminals forreceiving an analog signal, output terminals for supplying a clampedsignal value corresponding to an instantaneous value of the analogsignal, first and second direct current supply terminals, seriallyconnected diode means having first and second terminals and intermediateterminal means, a first resistor connected between the first directcurrent supply terminal and the first diode means terminal, a secondresistor connected between the second diode means terminal and thesecond direct current supply terminal, a switching tube having a currentconducting electrode connected to the first diode means terminal, acurrent conducting electrode connected to the second resistor, and acontrol electrode, a second switching tube having a current conductingelectrode connected to the first resistor, a current conductingelectrode connected to the second diode means terminal and a controlelectrode, a terminal for receiving trigger impulses coupled to saidcontrol electrodes, cathode follower means connected to the first andsecond resistors having a control electrode and having a cathoderesistor connected to the said output terminals, the control electrodeof said cathode follower means being connected to one of the terminalsof the diode means.

6. A recycling clamp comprising in combination input terminals forreceiving an analog signal, output terminals for'supplying a clampedsignal value corresponding to an instantaneous value of the analogsignal, first and second direct current supply terminals, seriallyconnected diode means having terminals comprising first and secondterminals and intermediate terminal means, a first resistor connectedbetween the first direct current supply terminal and the first diodemeans terminal, a second resistor connected between the second diodemeans terminal and the second direct current supply terminal, aswitching tube having a current conducting electrode connected to one offirst and second diode means terminals, a current conducting electrodeconnected to one of the resistors and having a control electrode,terminal means for receiving trigger impulses coupled to said controlelectrode, coupling means between the said output terminals, and one ofthe terminals of the diode means, and the analog input terminals beingcoupled to one of the other terminals of the diode means.

7. A clamping circuit comprising a diode array,

first and second resistors connecting respective ends of i the array tofirst and second points of fixed potential,

a condenser connected to said array and to one of said resistors forproviding an output signal,

an input signal terminal connected to the array, and

means for causing the signal on the condenser to momentarily follow a'signal on the input terminal comprising first switch means forselectively shunting said array and said first resistor, second switchmeans for selectively shunting said array and said second resistor, andmeans for simultaneously and momentarily operating both said switches toshunt both resistors and said array.

References Cited in the file of this patent UNITED STATES PATENTS2,438,947 Rieke et al. Apr. 6, 1948 2,443,195 Pensyl June 15, 19482,990,477 MacIntyre -a June 27, 1961

7. A CLAMPING CIRCUIT COMPRISING A DIODE ARRAY, FIRST AND SECONDRESISTORS CONNECTING RESPECTIVE ENDS OF THE ARRAY TO FIRST AND SECONDPOINTS OF FIXED POTENTIAL, A CONDENSER CONNECTED TO SAID ARRAY AND TOONE OF SAID RESISTORS FOR PROVIDING AN OUTPUT SIGNAL, AN INPUT SIGNALTERMINAL CONNECTED TO THE ARRAY, AND MEANS FOR CAUSING THE SIGNAL ON THECONDENSER TO MOMENTARILY FOLLOW A SIGNAL ON THE INPUT TERMINALCOMPRISING FIRST SWITCH MEANS FOR SELECTIVELY SHUNTING SAID